This invention relates to the field of fuses, more particularly time delay dual element cartridge fuses capable of interrupting circuits under both overload and short circuit conditions.
Time delay fuses include a short circuit element projecting from a spring trigger mechanism to form a fusing assembly, which is then held in an insulated tube and mechanically and electrically connected to opposed end ferrules. A time delay low voltage cartridge fuse of this type is disclosed in U.S. Pat. No. 4,344,058, Knapp, Jr , et.al.
The short circuit element is anchored to the spring trigger mechanism through a spring loaded bullet member. The bullet member is a cold headed part and extends outward from a barrel having a spring therein to bias the bullet toward re-entry of the barrel. An open ended paper sleeve surrounds the barrel, and the open end of the barrel opposite the bullet is covered with a barrel plug which is received in an inner radial lip therein. The barrel also has an outer radial flange on its open end adjacent the inner radial lip. This flange is used to position the open ended paper sleeve thereon. The paper sleeve is assembled over the barrel and biased to abut the flange. The paper sleeve insulates the heater strip, which carries current from one end of the fuse, from contact with the barrel. Since the sleeve is assembled to abut against the outer radial flange, and the heater strip is soldered to the end of the bullet and extends over the paper sleeve, the paper sleeve must be assembled to the barrel before the heater strip is soldered to the bullet. The paper sleeve insulates the trigger barrel from the heater strip.
To help isolate the interior of the trigger mechanism, a barrel plug is located in the end of the barrel and an insulating washer is disposed over the barrel plug and end of the trigger. The barrel includes a circumferential ledge on its inner diameter adjacent its end into which the barrel plug is located. The barrel plug helps isolate the inside of the trigger from arc quenching fillers, such as sand, disposed within the fuse. The washer electrically insulates the barrel plug from the end ferrule. To mechanically and electrically link the heater strip, bullet and short circuit element, a fusing alloy is disposed at the juncture of the bullet and barrel, heater strip and short circuit element. The fusing alloy is a low melting point solder designed to melt when the heat given off by the heater strip from a long term overload condition elevates its temperature to the melting point. Upon assembly of this sub-assembly into the insulating tube, the heater strip is folded over the sides of the barrel, and a ferrule is fitted and heated over the end, creating an electrical path therebetween.
To quench any electrical arcing which arises after the fuse element melts, some prior art fuses are packed with arc quenching sand. The washer and barrel plug are used to prevent entry of sand into the trigger mechanism, and the bullet portion is tapered to allow a minimum space between it and the barrel when the fuse is in the closed position, which helps prevent jamming of the trigger with the filler as the trigger opens. The sand surrounds both the trigger and short circuit strip elements of the fuse.
In operation, the fuse will open under two types of conditions. If a short circuit is encountered, the heat produced in the short circuit element, which is caused by the passage of excess electric current through the necked down portions thereof, causes the short circuit element to melt, opening the circuit across the opposed end ferrules of the fuse. Under long term overload conditions, the electric current flowing through the heater strip generates heat, and after a sufficient period of time the heat will cause the fusing alloy to melt. This causes the spring to retract the bullet into the barrel, thus causing the bullet to pull away from the short circuit element, opening the circuit through the fuse.
In the prior art designs, the spring loaded trigger is a complex and a costly design The barrel is made on a screw machine, to include the inner radial lip to retain the plug and an outer flange to locate the paper sleeve. The fuse plug and paper sleeve must be separately located onto the barrel, and the washer must be placed over the trigger, before the ferrule is loaded over the end of the tube.